This invention pertains to a method and apparatus for controlling the drying of a product, such as lumber, in apparatus such as a dry kiln. More particularly, the invention proposes what might be thought of as a "hands off" method and apparatus for the continuous following of the moisture level and of the moisture removal rate respecting such products. A preferred embodiment of, and method of practicing, the invention are described herein in conjunction with the kiln-drying of lumber, in which situation the invention has been found to have particular utility.
As is well known to those skilled in the wood products industry, kiln-drying of lumber is an important operation. Typically, lumber which is to be dried is stacked on a cart which is wheeled into a kiln, wherein it is subjected to heat and continuous gas flow. "Wet bulb" and "dry bulb" temperatures are monitored within the kiln during drying--the difference therebetween being known as "depression". The amount of depression affects the rate of drying, and depression is manually controlled, when thought necessary, through the introduction of additional moisture into the kiln atmosphere, and/or through changes in the venting of the kiln.
At the beginning of a drying operation, a kiln operator, using a conventional moisture meter, manually checks the moisture content of the undried lumber at various selected points distributed throughout the charges of lumber in the kiln. Depending upon the species of lumber involved, the operator consults existing charts, that have been developed over the years, which indicate the rate at which moisture should be removed, and the end-point dryness which is desired. These charts also provide some rough indications of the times and temperatures involved in achieving this end-point at a proper drying rate. The kiln is then activated, and drying begins.
By watching wet bulb and dry bulb temperature data, the operator estimates moisture-removal rate, and manually makes any adjustments which he feels are necessary. However, the only way in which he can actually know how much moisture has been removed at any given time is to shut down the kiln, enter it, and take another series of distributed-point moisture readings. This, in fact, is the technique most widely used heretofore. Often, this kind of checking (which takes a considerable amount of time, and requires stopping the drying operation), is done well in advance of when the lumber has reached its end-point dryness. As a consequence, the kiln must be reactivated for another estimated time period.
Hopefully, a skilled operator will estimate fairly accurately when the lumber has reached proper dryness. However, it is usually the case that the dried lumber ends up somewhat more moist than what is optimally desired. Rather than take the chance of over-drying, the operator will usually early declare the lumber finished or ready. However, there are occasions, occurring often enough to be troublesome, where lumber becomes over-dried. When this happens the lumber must be downgraded, and this, of course, creates an economic loss.
A general object of the present invention is to provide a unique method and apparatus for continuously monitoring and controlling, or enabling controlling of, the drying of a product, such as a charge of lumber, in a manner offering a number of significant improvements and advantages over prior art techniques.
More specifically, an object of the invention is to provide such a method and apparatus which proposes a "hands off" technique for such monitoring and control.
A further object of the invention is to provide a method and apparatus as generally outlined which deals both with monitoring and controlling both the absolute moisture content of a charge, and the rate of removal of moisture from the charge.
According to the invention, during drying, the weight of a charge, such as a charge of lumber, is continuously monitored to indicate both total weight loss and rate of weight loss. These data are employed to enable automatic, or if desired, manual, adjustment of drying action so as to control the moisture removal rate, as well as to stop the drying action when the desired terminal dryness is reached. Such data are also used to produce a continuous chart recording of exactly what is occurring with respect to moisture removal from the charge. A special feature of the invention is that multi-point weight monitoring is used to provide localized moisture-content data respecting a charge. The data derived from this feature is used further to adjust drying action so as to promote uniform moisture levels throughout a charge during drying. In other words, it tends significantly to promote a uniformly dried product.
Yet another important object of the invention is to provide apparatus which may easily and quickly be installed in existing dry kilns, with little appreciable disruption of dry kiln operation.
Still a further object of the invention is to provide a method and apparatus as outlined which is simple, economical, accurate and extremely reliable.
These and other objects and advantages which are attained by the invention will become more fully apparent as the description thereof which now follows is read in conjunction with the accompanying drawings.